The coating of thermoplastic polymer substrates with moisture resistant barrier coating compositions to provide impermeability to gases such as oxygen, and liquids, is known [See, e.g., U.S. Pat. No. 3,282,729]. Alkali metal polysilicates have long been known as protective coatings that modify the permeability or surface characteristics of polymeric films and other articles [See, e.g., U.S. Pat. Nos. 1,007,482; 1,424,425; 1,949,914; 3,102,038; 3,130,061; 3,180,747; 3,492,137; 3,522,066; 3,533,816 and U.S. Pat. No. 3,706,603].
Lithium (Li), potassium (K) and sodium (Na) polysilicates are used in coatings for a variety of surfaces. For example, Hecht and Iler, Canadian Pat. No. 993,738, describe a gas and liquid-impermeable coating for polymeric substrates comprising lithium polysilicate having a mole ratio of SiO.sub.2 to Li.sub.2 O of about 1.6 to 4.6.
Titanium dioxide has been added as a pigment to certain coatings. For example, U.S. Pat. No. 2,998,328 refers to a finish or overcoat comprising (1) a reactive liquid component comprising an aqueous solution of an alkali metal silicate and (2) a pigmented blending component. The reactive liquid component dissolves an alkali metal silicate such as sodium silicate, potassium silicate, lithium silicate, or a mixture of such silicates in water. The pigmented blending component may be titanium dioxide. U.S. Pat. No. 3,379,559 refers to a glass container having a metal oxide coating over which is adhered a synthetic resin. The metal oxide may be titanium dioxide. U.S. Pat. No. 3,833,406 refers to a closed container having a desiccant coating applied to its inner surface. The coating may be titanium oxide and the container may be plastic.
U.S. Pat. No. 4,552,791 refers to a container made of an organic resin having improved vapor barrier characteristics. Plating materials may include titanium oxide. U.S. Pat. 5,494,743 refers to antireflective coatings, such as titanium oxide applied to plastic materials.
The refractive indices, n, of poly(ethylene terephthalate)(PET) and polypropylene (PP), both semicrystalline polymers, depend upon the degree of crystallinity, and, because both are birefringent, also depend upon the degree of orientation [J. Seferis, "Refractive Indices of Polymers", in Polymer Handbook, 3rd ed., ed. J. Brandrup and E. H. Immergut, pp. VI-453, Wiley, N.Y (1989)]. The phase average refractive indices, n, of the amorphous and crystalline regions of PET are 1.57 and 1.64, respectively. Biaxially-oriented PET, having .about.40 percent crystallinity, may be generally said to have a refractive index of approximately 1.6.
The phase average refractive indices, n, of amorphous and crystalline regions of PP are 1.52 and 1.47, respectively. Biaxially-oriented PP film may be generally said to have a refractive index of approximately 1.5.
The alkali metal silicates (M.sub.2 SiO.sub.3) of lithium (Li.sub.2 SiO.sub.3), potassium (K.sub.2 SiO.sub.3) and sodium (Na.sub.2 SiO.sub.3) have refractive indices, n, of 1.59, 1.52, and 1.52, respectively ["Physical Constants of Inorganic Compounds", in Handbook of Chemistry and Physics, 62nd ed., ed. R. C. Weast, pp. B-114, B-136, B-143, B-150, CRC Press, Boca Raton (1981)]. The refractive index of amorphous silica (lechatelierite) is 1.46 ["Physical Constants of Inorganic Compounds", in Handbook of Chemistry and Physics, 62nd ed., ed. R. C. Weast, pp. B-114, B-136, B-143, B-150, CRC Press, Boca Raton (1981)]. The refractive index, n, of a dried alkali metal copolysilicate coating of the formula (Li.sub.2 O).sub.x (K.sub.2 O).sub.1-x (SiO.sub.2).sub.y, wherein x [the mole fraction of Li.sub.2 O in the combined alkali metal oxides (M.sub.2 O)] is 0.5, and y [the SiO.sub.2 :M.sub.2 O mole ratio] is 3.64, can be calculated to be 1.49. Clearly a coating of approximately this composition is a better match to the refractive index of PP than that of PET.
Vapor barrier metal polysilicate coatings have been noted to have several disadvantages including discoloration and high refractive indices on some polymeric substrates. Although metal polysilicate barrier coating solutions are water clear, the present inventors have found that there is a faint color caused by thin film interference on coated PET substrates. On coated PET bottles this yields a slightly "oily" appearance that is unacceptable to customers.
There remains a need in the art of barrier coatings for new barrier coating compositions and methods which overcome these deficiencies and are widely available as vapor, gas and/or aroma barriers for polymeric articles, e.g. packaging products.